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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: December 8 The Shadow of the Earth
Message-ID: <146@utastro.UUCP>
Date: Sun, 8-Dec-85 02:00:32 EST
Article-I.D.: utastro.146
Posted: Sun Dec  8 02:00:32 1985
Date-Received: Mon, 9-Dec-85 03:52:18 EST
Organization: U. Texas, Astronomy, Austin, TX
Lines: 38

If you've got a clear sky at sunset, you can see the shadow of the
Earth.  We'll tell you how -- right after this.

December 8  The Shadow of the Earth

Like everything that has sunlight shining on it, the planet Earth casts
a shadow.  You can see Earth's shadow without traveling into space --
but first imagine how it would look if you were in space.

All the space inside our solar system is lighted by the sun.  The only
dark places are in the shadows of obscuring objects -- such as planets
or moons.  Like people sitting around a campfire, each object in the
sun's family has one lighted side and one dark side -- and long shadows
extend out opposite the source of light.

We call Earth's lighted side its "day side." As the planet rotates, the
day side continually moves around it.  The trick to seeing Earth's
shadow is to be standing outdoors just when the day side is moving away
-- in other words, at sunset.

You need a couple of other things for this observation including a
cloud-free horizon and a clear view of the east.  If conditions are
good, you can look opposite the direction of the sunset to see a curved
line of darkness moving up from the eastern horizon.  You've probably
seen this ascending darkness many times and imagined that it was haze
or clouds.  What you're really seeing, though, is the shadow of the
Earth.  The shadow is curved because Earth is round -- and you can see
it because its deep blue color contrasts with the pink and pale blue of
the twilight sky.  Watch for it -- the shadow of the Earth -- after
sunset some evening soon.



Script by Deborah Byrd.



(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin